The present invention relates to a novel process for the sulphoxidation of n-paraffins using sulphur dioxide and oxygen under irradiation with UV-containing light in the presence of an amount of water which is appropriate to ensure that the sulphoxidation mixture is in homogeneous phase.
Among the various process variants for the sulphoxidation of n-paraffins, the most advantageous from the points of view of economy and quality is the so-called light-water process (Chemie in unserer Zeit 13 (1979), 157). The addition of water allows the paraffinpersulphonic acid which is produced as an intermediate to react with sulphur dioxide giving paraffinsulphonic acid and sulphuric acid. The amount of water added is normally appropriate to ensure that a two-phase system is present during the sulphoxidation.
DE-C-910,165 discloses a further variant of the sulphoxidation with water addition, in which paraffin and water are emulsified by means of a very high throughput of the reaction gases. However, in this variant, the water is not ideally distributed. This has a deleterious effect on the undesired formation of paraffindisulphonic acid which, for quality reasons, must be limited to 10% by weight. This limitation allows only a very low parafin conversion with disadvantages with regard to space-time yield. In this respect, H. G. Hauthal, Alkanesulfonates (1985) mentions a conversion of 1%. The previously cited DE-A-910,165 mentions a conversion of about 0.6%, and EP 282,962 mentions a figure of about 1.5%.
DE-C-910,165 and DE-B-1,910,860 refer to the amount of water added. According to these documents, the amount of water added to the reaction mixture must be determined in such a way that the concentration of the resulting sulphuric acid in the aqueous phase does not increase beyond 20%; no justification for this statement is given in the abovementioned documents.
The object of the invention is therefore to provide a process for the sulphoxidation of n-paraffins using sulphur dioxide and oxygen under UV-irradiation and with the addition of water, in which the reaction gives paraffinsulphonic acid with high selectivity under industrially more favourable conditions and also at higher conversion.
Surprisingly, in the sulphoxidation, a very narrow, conversion-dependent range of water concentration has been found within which the sulphoxidation mixture is present in the form of a water-white, homogeneous reaction solution. Advantageously, within this range, paraffin is converted to paraffinsulphonic acid with high selectivity at significantly higher conversion.